This invention relates generally to removal of laser ablated particles in imagesetters and platesetters for the prepress printing industry, and more specifically to a vacuum system for removing the ablated particles away from a film or printing plate immediately after imaging thereon with an electromagnetic waveform.
In the prepress printing industry, it is well known that a substrate characterized as either a film or a printing plate (hereinafter jointly referred to as a "plate") can have an image transferred thereto by selectively "burning" sections of a thermally-sensitive surface of the plate with an electromagnetic waveform. This method of imaging a plate is generally referred to as thermal imaging. Typically, the power necessary for such image transfer is attained through the use of a laser light source for emitting the electromagnetic waveform. The specific chemical makeup of the plate will dictate the required characteristics of the light source which are necessary to adequately bum an image into the plate at the required depth. Alternatively, a plate can be manufactured having the appropriate chemical makeup to allow imaging with a predetermined light source.
In an internal drum imagesetter or platesetter (hereinafter jointly referred to as "platesetter"), a plate is positioned along the internal cylindrical surface of the drum prior to imaging. The drum and surrounding components create an internal drum chamber. The air space above the plate and within the imager is closed within the internal drum chamber to prevent contamination of the plate, the internal surface of the drum, optics and other components from dust, dirt and other contaminants.
When a laser beam is transmitted to the thermally-sensitive surface of the plate positioned for imaging within the platesetter, laser ablation occurs. Laser ablation refers to a high-yield photon sputtering process which effectively removes material from the thermally-sensitive surface of the plate. The material effectively explodes from the surface of the plate, resulting in a gaseous plume of smoke and debris. The ablated materials will thereafter disperse throughout the air in the internal drum chamber and will settle onto the plate, the internal drum surface, optics and other components touching the air space of the internal drum chamber. Laser ablation and plume formation is discussed in detail, for instance, in "Laser Ablation And Desorption" edited by John C. Miller and Richard F. Haglund, Vol. 30, 1998 by Academic Press, herein incorporated by reference in its entirety to provide supplemental background information on laser ablation which is helpful but not essential in appreciating the applications of the present invention.
U.S. Pat. No. 5,574,493 issued Nov. 12, 1996 to Sanger et al. describes a vacuum collection system for use to remove ablated materials from an external drum imager which uses a dye-ablation printing process. The system includes a cylindrical lens barrel which carries an imaging lens system for a laser and a vacuum tube attached to the lens barrel. The vacuum tube is positioned so as to be on the lateral side of an orifice box away from material previously written. This draws the ablated material over unwritten portions of the medium and reduces the problem of blow-back of contaminates onto the previously written surface. In this system, if ablated material is drawn over a previously written surface, a substantial portion of the ablated materials (i.e. blow-back) will stick to the medium. Sanger et al. also teaches that build-up of ablated materials in the vacuum chamber is inhibited by either applying heat or a solvent to the vacuum chamber.
Sanger's method is limited to use with an external drum imager with a rotating drum, whereby both the laser system and the vacuum collection system are stationary. Moreover, the vacuum tube for removing ablated particles precedes the laser along the imaging path, so that the area of the medium to be imaged is cleaned by vacuuming prior to imaging.